This invention relates to the production of magnetic labels or tags which can be used, for example, in identifying articles to which they are attached.
In previous patent applications (PCT/GB96/00367, now published as WO97/04338; and PCT/GB96/00823, now published as WO96/31790) we have described remote identification tags which may be interrogated using alternating magnetic fields. In response to interrogation, the tags emit magnetic signals which may be detected in suitable receiving equipment. Such tags have many applications in diverse areas including article identification, security and access control.
Certain of the tags described in International Patent Application PCT/GB96/00367 (claiming priority from GB 9506909.2 and GB 9514581.9) contain elements of high permeability, low coercivity, magnetic alloy in the form of a strip, wire or thin film, overlaid with a medium coercivity magnetic layer. The magnetic signal or "signature" generated by the high permeability element in response to interrogation by an applied magnetic field is determined by the magnetisation pattern stored in the medium coercivity layer, which acts as a "coding" layer, and by the characteristics of the high permeability element.
In practical implementations, tag coding can be carried out during tag manufacture by contact-recording the appropriate magnetic pattern onto a continuous coding layer using an array of permanent magnets. This method is particularly suitable for producing many tags with the same code.
Another method, which is suitable for manufacturing tags whose code will never require to be changed, is to place magnetised pieces of coding material at appropriate positions on the high permeability material.
A more flexible arrangement uses a magnetic recording head of the type well-known for recording information onto magnetic tape. With this arrangement, tags manufactured with an un-magnetised coding layer can have patterns individually coded at the point-of-issue. This is extremely useful for applications such as airline baggage tagging, where individual tag details (passenger name etc.) are not known in advance of issue. The drawbacks to this method are the need for precise alignment of the head with the tag, and the cost and complexity of the equipment. It is also difficult to achieve very high recording field strengths, so tags with high resistance to corruption by extraneous magnetic fields are hard to produce.